A. Field of the Inventions
The present invention relates to the art of coatings for products requiring a substantially clear surface. Specifically, the invention is a solution to the problem of point lumps in the coatings of, for example, optical fibers, by regulating the filtering conditions, such as pore size, pressure drop and temperature during filtering of the coating.
B. Background Discussion
Optical fibers for light transmission as drawn are exceptionally strong and have very few intrinsic defects. However, even a small surface flaw can render such a fiber brittle and easily broken. Thus, such fibers are generally coated by a primary and optionally a secondary coating for protection as disclosed by U.S. Pat. Nos. 6,014,488, 5,352,712, 5,527,835, 5,538,791, 5,587,403 and 6,048,911 to Shustack, each of which is incorporated herein by reference in its entirety.
In the manufacture of optical fiber cable, a glass preform rod, generally manufactured in a separate process, is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and an optical fiber is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower.
In order to protect the drawn optical fiber from damage caused by abrasion or handling, the fibers are traditionally protected by a coating. The coating is applied before the drawn fiber reaches any rolling machine or capstan to limit any defects which may result during rolling thereof.
Other than merely protecting the surface of the optical fiber from abrasion, the structure of the coating is selected to limit transmission defects. Specifically, as described in U.S. Pat. No. 4,962,992 to Chapin et al., herein incorporated by reference in its entirety, sub-standard coatings can result in large bubbles, voids, non-concentric coatings and microbending, each of which will often cause transmission errors.
Dual coated optical fibers typically are used in cables to obtain design flexibility and improved performance, while protecting against the aforementioned undesired effects. Typically, a dual coated optical fiber includes a coating system having an inner or primary coating layer characterized by a relatively low modulus rubbery material applied to the optical fiber. The modulus of the primary coating should be effective in reducing the stress transmitted to the glass by an external lateral force, thus reducing microbending of the glass. Primary coating materials have been characterized by an equilibrium modulus of elasticity in the range of about 50 psi to 200 psi. Equilibrium modulus may be defined as the final modulus that a cross-linked material will reach in time or at high temperatures. This modulus is chosen so that the primary coating achieves its principal purpose, i.e., the attenuation and uniform distribution of stress supplied to the fiber. Through this attenuation and distribution, losses due to microbending are substantially reduced. The purpose of the primary coating is to allow for a limited degree of bending without creating microbending attenuation errors.
Although the coatings do not play a direct role in the signal transmission through the glass fiber, the coatings are critical to the fiber performance. The coatings are used to provide (1) strength retention; (2) environmental protection; (3) microbending loss resistance; as well as (4) assist in fiber identity and (5) space individual fibers when bundled into cable structures.
The primary coating is usually soft or rubbery. Because the primary coating has a low glass transition temperature after curing (e.g., −20° C. to −50° C.), it has a low modulus and functions as a shock absorber. The primary coatings also generally have excellent adhesion to glass under a variety of environments and operating conditions, while being low enough to permit easy coating strippability, while resisting delamination. Such strippability is described in U.S. Pat. No. 6,014,488 to Shustack, herein incorporated by reference in its entirety.
An outer or secondary coating layer is applied over the primary layer. The secondary coating is usually of a higher modulus material to provide abrasion resistance and low friction for the coated fiber. The dual coating materials serve to cushion the optical fiber by way of the primary layer and to distribute the imposed force by way of the secondary layer, so as to isolate the optical fiber from bending moments.
The secondary coating is generally a hard, scratch resistant coating which may have a glass transition temperature as high as or greater than approximately 80° C. The secondary coating is particularly selected to exhibit high modulus and low elongation and to provide environmental protection against, for example, harsh mechanical and chemical conditions. The coefficient of friction cannot be too low (slippage of the fiber) or too high (tackiness or stickiness). It is also desirable to maintain a suitable cure degree range to limit slippage and tackiness of the coating.
Generally, the primary and secondary coatings are cured (crosslinked) by ultraviolet light. However, other curing techniques include electron beam, or electromagnetic radiation, including heat and visible light.
Typical coated optical fibers have a total diameter of approximately 245 microns. The drawn glass fiber core typically has a diameter of approximately 8 microns, with a concentric cladding of approximately 117 microns. The primary coating typically has a thickness of approximately 65 microns, with the outer coating of 55 microns forming the balance of the thickness of the coated fiber.
The innermost structure or glass fiber is designed to carry the signal from station to station. Typically, a germanium and/or erbium doped glass core is surrounded by a cladding derived from pure glass. The refractive index of the glass core is greater than that of the cladding. This ensures converging of signals as well as confining the light beam to the center of the core.
The ultraviolet-curable optical fiber coatings generally include an oligomer, a monomer, a photoinitiator and additives. Typical oligomers, monomers, photonitiators and additives are disclosed by U.S. Pat. Nos. 6,014,488 and 5,352,712 to Shustack, each of which is incorporated herein by reference. The oligomer has a high viscosity and provides the basic properties of the coating, while the lower viscosity monomer helps to adjust the crosslink density or assist in adjusting viscosity. The photoinitiator is used to start the curing reaction, and the additives are included to modify properties such as adhesion (e.g., silicone coupling agents), storage stability (e.g., shelf file) and coefficient of friction.
The glass portion of the optical fiber is conventionally manufactured via a draw tower. A consolidated blank or preform is lowered into a furnace set between 1900° C. and 2300° C., and a high precision computer controlled track is used to draw the fiber from the molten glass preform. The bare glass fiber cools down to 50° C.-60° C. as it traverses down the tower. The coating is applied to the bare fiber as it is drawn at around 35° C. to 40° C. for draw speeds over 800 meters/minute. For higher speeds, further adjustments may be needed, depending upon viscosity of the coating.
The coatings may be applied in a traditional wet-on-dry process or a modern wet-on-wet process. In the wet-on-dry process, uncured primary coating is applied to the bare fiber. Then the primary coating is at least partially cured via a set of UV lamps. Once this curing occurs, the secondary coating is applied and cured by its own set of UV lamps. The secondary coating is applied, in a “wet” state, onto the “dry” primary coating.
An alternative to the wet-on-dry process is a wet-on-wet coating process. In the wet-on-wet process, the primary coating is applied before the secondary coating. However, the primary coating is not cured until after the secondary coating is applied. Thus, both coatings are cured simultaneously. The particular design of a coating apparatus used in wet-on-wet coating is described in U.S. Pat. No. 4,474,830 to Taylor, herein incorporated by reference in its entirety. Because only a single set of UV lamps is required in a wet-on-wet application system, a smaller tower can be used and run at a faster rate than that of the wet-on-dry process.
Conventionally, the coating compositions are filtered prior to application to the glass fiber. For example, nylon filters with pore sizes of 0.1-5.0 microns are used, in a filtering process run at approximately 105° F. (40° C.) and with a pressure drop (ΔP) of approximately 40 psig (275 kPa) to approximately 60 psig (415 kPa).
However, coated optical fibers have been known to have imperfect coatings, even with this filtering step. Generally, imperfections in the coatings may result from process defects (i.e., the process used to coat the glass fibers) or composition defects (i.e., the materials and compositions forming the coatings). Process defects include, for example, concentricity, while composition defects include, for example, fibrous inclusions or other contaminants. Additionally, defects, such as bubbles or delamination, may be a process defect, a composition defect or combination thereof.
One type of composition defect is a point lump. The nature of point lumps has previously been unknown. However, the present inventors believe point lumps to be caused by high molecular weight fractions of gel-like formations in the secondary coating (hereafter referred to as “gels” or “gel formations”).
Point lumps may lead to (1) a coated fiber exhibiting a low tensile strength, due mainly to mechanical obstruction in post draw processing (e.g., color inking), or (2) a defect on the coating which can lead to microbending and attenuation. These gel formations may be visually perceptible or may require magnification. Most often however, the point lumps cause a shift of 5 microns (or approximately 2% of the overall diameter) in the diameter of the entire fiber structure. Additionally, while the presence of point lumps may not reduce the tensile strength of the coated fiber, the point lumps may get caught in a post-process coloring die step.
Often the point lumps have the same refractive index as the cured coating. Thus, they appear to be made of the same materials even though they are different. FIG. 1 shows an optical fiber 110 without any point lumps. It has a glass fiber 112, a primary coating 114 and a secondary coating 116. One type of point lump, an external point lump, causes a change in the total diameter of the coated fiber. External point lumps 128, indicated in FIG. 2, are generally visible because even if the refractive index of the lump is the same as the coating, at least the “bulge” is observable. FIG. 2 shows a portion of an optical fiber 120 having a glass fiber 122, a primary coating 124, a secondary coating 126 and an external point lump 128. FIG. 3 shows a portion of a optical fiber 130 having a glass fiber 132, a primary coating 134, a secondary coating 136, and an internal point lump 138. Internal point lumps often are not visible to the naked eye. Internal point lumps generally cause compression of the primary coating such that the total diameter of the coated fiber does not change, only the relative diameters of the individual components is altered.
The presence of point lumps is a problem. Due to the competitive nature of the optical fiber industry, it is not economical for optical fiber producers to “scrap” or discard product containing such imperfections. When optical fiber technology was in its earlier stages, and costs of producing coated optical fiber were much greater than today, the cost of scrap was less of an issue. Compounding the problem is the fact that the point lumps are not detectable until after the coatings have been applied to the glass fiber.
Adding to the difficulties encountered in preventing the inclusion of point lumps, is the testing therefore. Unfortunately, the only known way to test for point lumps is to draw fiber from a preform and coat the drawn fiber. Only after the coatings are applied to the glass fiber, can the presence of point lumps be detected. Even then, no known automatic testing device has been developed for isolating point lumps. Devices, however, do exist to determine a total number of defects present, as long as certain conditions are met, but the results do not specifically determine the number of point lumps present. Therefore, the only known method for determining the number of point lumps in a coated optical fiber is to manually and microscopically scan the surface thereof and count. Thus, the testing process is time consuming and expensive.
It is conventionally known to filter uncured secondary coating material at the coating manufacturing plant. However, this was done at high pressures to speed production and to remove microscopic, yet visible, particulates. This filtering is not performed to remove any invisible gels, believed to be the source of the point lumps.
Additionally, the same types of problems, i.e., the inclusion of point lumps, have been known to cause the same unwanted point lumps in other products requiring a coating, such as compact disks, dental compositions and laminated surfaces.
Therefore, there exists a need in the art to provide a method to economically produce an optical coating with little or no point lump defects resulting from the elimination of gels or gel formations in the coatings prior to application on to the bare fiber.